pci_bridge: Use a default map_irq function

The PCI bridge spec defines a default swizzle for translating INTx
IRQs from secondary bus to primary.  Use this by default for any
bridge that doesn't set a function.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
pci_bridge: Remove duplicate IRQ swizzle function

pci_bridge_dev_map_irq_fn() is identical to pci_swizzle_map_irq_fn(),
which is now the default for all PCI bridges.  We can therefore remove
this function and the pci_bridge_map_irq() call that used it.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>